1. Field of the Invention
The present invention relates to an optical system for defecting a surface defect and a surface defect tester and, particularly, the present invention relates to an optical system for use in a detection of surface defect of a flat plate such as a magnetic disk or a glass substrate thereof to detect the size of irregularity of a surface of the flat plate with high precision and the surface defect tester using the same optical system.
2. Description of the Prior Art
A magnetic hard disk used as a recording medium of a computer system is tested for surface defect and size of the surface defect in a substrate state or in a complete magnetic disk state in which a magnetic film is painted thereon.
The size of the recent magnetic disk is 3.3 inches or smaller and the recording density thereof is substantially increased by the employment of a giant magneto-resistance (GMR) head. In such magnetic disk, a glass substrate, which has thermal expansion coefficient smaller than that of the conventional aluminum substrate and is as thin as in a range from 0.6 mm to 0.8 mm, is used.
FIG. 5(a) shows a main construction of a conventional surface defect tester 10 for magnetic disk.
The surface defect tester 10 shown in FIG. 5(a) is constructed with a rotary mechanism 2, a detection optical system 3 and a surface defect detection/processor 4. A disk 1 to be tested is mounted on a spindle 21 of the rotary mechanism 2 and rotated by a motor (M) 22. On the other hand, the detection optical system 3 is constructed with a light illuminating system 31 including a laser light source 311 and a condenser lens 312 and an optical light receiving system 32 including a condenser lens 321 and a light receiver 322. A laser beam LT produced by the laser light source 311 is condensed by the condenser lens 312 to a laser spot Sp on a surface of the disk 1.
When the disk 1 is moved horizontally while being rotated, the laser spot Sp moves in a radial direction R of the disk 1, so that a surface of the disk 1 is scanned spirally. In this case, in order to make a total scan time of the disk 1 as short as possible, an area of the laser spot Sp is made ellipsoidal having a length xcfx861 in a minor axis direction and a length xcfx862 in a major axis direction as shown in FIG. 5(b) and the major axis is set perpendicular to the scan direction to increase a scan width of the laser spot. The laser spot Sp is scattered by a defect F of the surface of the disk. A reflected light SR is condensed by the condenser lens 321 of the optical light receiving system 32 and the condensed light is received by the light receiver 322 including the optoelectric converter element such as, for example, an avalanche photodiode (APD) or a photo multiplier tube (PMT). An output signal of the light receiver 322 is inputted to a signal processing circuit 41 of the surface defect detection/processing unit 4. The defect F is detected by a defect detection signal output from the signal processing circuit 41. The size of the defect F is detected or calculated according to a level of the detection signal outputted from the light receiver 322. The signal processing circuit 41 for detecting the defect and classifying or calculating the size of detected defects by the so-called sampling includes an amplifier for amplifying the output signal of the light receiver 322, a sampling circuit for sampling peak values of the amplified output signal, which corresponds to a defect and is larger than noise component of the output signal with a pulse supplied from a rotary encoder 23 to detect the peak values of the sampled output signal, an A/D converter for digitizing the sampled peak value and a position data producing circuit responsive to the pulse signal from the rotary encoder 23 for producing a position data on the disk, etc.
The size data of the respective defects and the position data of the detects on the disk are converted into digital data by the signal processing circuit 41 and inputted to the data processor 44 composed of an MPU 42 and a memory 43, etc. The number of defects of each size are counted by the data processor 44 and the size data and the count value of the defects, etc., are outputted to a printer (PR) 45 together with the position data of the defects on the disk 1. In this case, these data may be printed out as a map on the disk. Alternatively, the defect size is displayed on a display (CRT) 46, etc., together with the position thereof on the disk and the count value of the defects is displayed separately.
Incidentally, the rotary encoder 23 is provided in the vicinity of a rotary shaft of the motor 22 or in engagement therewith, detects an amount of rotation of the disk with reference to a reference position provided on the disk and sends a pulse signal corresponding to the amount of rotation of the disk to the signal processing circuit 41.
In order to clearly detect the size of recessed defect and protruded defect, the surface defect tester 10 optimally sets factors related to the detection sensitivity, such as illumination angle xcex8T of the laser beam LT of the optical illuminating system 31, light receiving angle xcex8R of the light receiving system 32, voltage V applied to the light receiver (APD) 322 or gain of the amplifier provided in the signal processing circuit 41, threshold voltage E for removing noise and laser output of the laser light source 311, etc., through the control panel 47. Incidentally, the detection sensitivity is regulated by using, as a sample disk, a practical disk having recessed defects such as dish-like defect, pit-like defect or scratch: defect having known size or a practical disk having protruded defects having a specific height.
JP H10-325713A assigned to the assignee of this application and belonging to the same technical field as that of the above mentioned prior art discloses a technique titled xe2x80x9cSurface Defect Test Method and Surface Defect Testerxe2x80x9d, in which a defect test is performed by using a sensitivity calibration disk having a plurality of radially extending dummy defect rows each including a plurality of protruded or recessed defects whose sizes are changed in steps: and the detection sensitivity is regulated correspondingly to the dummy defect row including defects whose sizes are increased or decreased in steps, by displaying the dummy defect row as a result of the test.
However, in such conventional defect detection system for disk in which a recessed defect of a disk or a protruded defect including extraordinary substance, etc., on the disk is detected by comparing a level of reflected or scattered light detected by a light receiver with a reference level, the size of the defect is detected as a level of received light, so that the detected defect size becomes inaccurate. Particularly, for the recessed defect or the protruded defect, depth or height of the defect influences the level of received light, so that the detection accuracy of defect size, which is an area extended in a flat plane, is low.
The sensitivity calibration disk used in this tester corresponds to U.S. Pat. No. 5,975,027, the content of which is shown in FIGS. 5(a) and 5(b).
Recently, it is required to improve the preciseness of defect configuration measurement and the preciseness of defect classification. However, it is impossible to precisely perform the classification of defects by the above mentioned prior art.
In order to solve this problem, JP H11-358769A assigned to the assignee of the present application discloses a technique in which a sensor arrangement including a plurality of APD elements is used as the light receiver 322 and a zigzagged stripe pattern corresponding to the APD elements is provided in front of the sensor arrangement. The recessed defect and the protruded defect are detected on the basis of a difference in amount of received light between adjacent APD elements of the sensor arrangement.
However, this technique requires the zigzagged stripe pattern and a number of detection circuits are necessary to detect the differences in light amount between adjacent sensor elements.
An object of the present invention is to provide a defect detecting optical system capable of precisely detecting the size of recessed or protruded defect on a surface of a flat plate and a surface defect detector using the same defect detecting optical system.
Another object of the present invention is to provide a surface defect detector capable of precisely detecting the depth or height of recessed or protruded defect on a surface of a flat plate and capable of easily classifying defects.
In order to achieve the above objects, a defect detecting optical system and a surface defect detector according to the present invention is featured by comprising a light illuminating system for emitting light beam having a width in a direction perpendicular to a main scan direction to relatively scan a surface of a flat plate and a light receiving system having a light receiver including a plurality (n) of light receiving elements arranged in the direction perpendicular to the main scan direction for picking up an image of a portion of the flat plate at a scan position, wherein, when a width of the image in the main scan direction is equal to or smaller than the width of the light receiving element and a certain one of the light receiving elements receives light reflected from the recessed or protruded defect, the reflected light is swung in the width direction of the certain light receiving element, which is perpendicular to the arranging direction of the light receiving elements, so that an amount of light received by the certain light receiving element is at least reduced.
With this construction of the defect detection optical system, which comprises the light illuminating system for emitting light beam having width in the direction perpendicular to the main scan direction to relatively scan the surface of the flat plate and the light receiving system including the n light receiving elements arranged in the direction perpendicular to the main scan direction for picking up an image of the flat plate at a scan position and in which, when the width of the image in the main scan direction is equal to or smaller than the width of the light receiving element and the certain one of the light receiving elements receives light reflected from the recessed or protruded defect, the reflected light is swung in the width direction of the certain light receiving element causing the amount of light received by the certain light receiving element to be at least reduced, it is possible to obtain a detection signal having magnitude, which is large when there is no defect on the surface of the flat plate and small when there is a defect thereon.
Since it is usual that there are a pair of slanted side faces in the recessed or protruded defect in the main scan direction, two detection signals are obtained for one defect. Therefore, it is possible to easily detect the size of the defect at that position on the basis of a distance between the two detection signals. Particularly, when n light receiving elements are arranged in a radial direction R, a plurality of light receiving elements arranged to cover an area of an image of a defect in the radial direction can detect the defect simultaneously. Consequently, it is possible to easily calculate the area of the defect.
Further, it is possible to easily determine the continuity of defect on the basis of a relation between defect detection coordinates assigned to the n light receiving elements determined correspondingly to the scan, respectively, and the detection signals from the respective light receiving elements. According to the continuity determination, it is possible to calculate an area of a defect among the recessed and protruded defects, which is somewhat deformed. Further, it is possible to precisely detect the depth of recessed defect or the height of protruded defect by averaging the levels of the two detection signals.
As a result, it is possible to realize the defect detecting optical system, which is capable of precisely detecting the size of the recessed and/or protruded defect on the surface of the flat plate and, further, precisely detecting the size and depth or the size and height of the recessed or protruded defect on the surface of the flat plate, and to realize the surface defect tester, which is capable of easily classifying defects.